Electrode contact for a piezoceramic actuator and method for producing same

ABSTRACT

A piezoceramic actuator includes a monolithic stack of thin piezoceramic films having internal electrodes arranged between the films, and rail-like elements electrochemically shaped onto the internal electrodes on outer sides of the stack. The internal electrodes can thereby be interconnected, via a suitable conductive element, in electrically conductive fashion at a certain distance from the side edges of the piezoceramic films so that these films form a continuous planar electrically conductive ribbon.

FIELD OF THE INVENTION

The present invention relates to a piezoceramic actuator, substantiallymade up of a sintered monolithic stack of thin piezoceramic films havinginternal electrodes, arranged between the films, that are electricallyinterconnected on the outer side of the stack to form at least twoelectrode groups, electrically separated from one another, havingalternatingly successive internal electrodes of the at least two groups.

BACKGROUND OF THE INVENTION

Piezoceramic materials have the property of becoming electricallycharged when impinged upon by mechanical forces, i.e. in particularunder mechanical compression or tension. On the other hand, the resultof an electric field applied to the piezoceramic material is that thematerial is mechanically distorted, i.e. expands or contracts.

These latter effects are utilized in actuators in order to performpositioning motions.

As a result of the construction of the actuator from a stack ofpiezoceramic films having a corresponding number of internal electrodes,a high electric field strength can be achieved within the piezoceramicfilms even with a limited electrical operating voltage, since in thecase of two electrode groups, the operating voltage is present betweeneach two adjacent internal electrodes.

Contacting of the internal electrodes can present practicaldifficulties. In conventional actuators, side regions of the stack thatare separated from one another are metal-coated in such a way that theone coating is electrically connected to the internal electrodes of theone group, and the other coating is electrically connected to theinternal electrodes of the other group.

Upon operation of the actuators, these metal coatings are exposed toconsiderable mechanical stresses when the actuator expands or contractsin accordance with the particular operating voltage. Large alternatingstresses can occur in this context if the operating voltage isfrequently switched on and off or is switched over in terms of itspolarity.

These mechanical stresses in the metal coatings can result in cracks inthe coating, with the consequence that a variable number of internalelectrodes can no longer be connected to the operating voltage sourceand the adjacent piezoceramic films cannot, or essentially cannot,contribute further to the work of the actuator.

It is therefore proposed in German Patent 196 48 545 A1 to cover theaforesaid metal coatings with a further electrically conductive layerthat is mechanically particularly flexible, in order to keep thefragments of the aforesaid coating which in German Patent 196 48 545 A1are also referred to as the “base metallization”—continuously inelectrically conductive contact with one another. This additionalcoating can, for example, take the form of a knitted or braided wirestructure, or that of a metal foam or corrugated sheet.

SUMMARY OF THE INVENTION

The present invention provides a piezoceramic actuator which has a stackof thin piezoceramic films having internal electrodes arranged betweenthe films, each of the internal electrodes extending, at least on aregion of the outer side of the stack, by way of rail-like or tab-likemetal elements which preferably can be formed by electrolyticallydeposited metal. The internal electrodes can thereby be interconnectedin electrically conductive fashion at a certain distance from the sideedges of the piezoceramic films, for example by way of optionallycorrugated metal films, knitted metal structures, or the like, orconductive plastic films, for example silicone films into whichelectrically conductive particles are embedded, so that these films forma continuous planar electrically conductive ribbon.

The rails or tabs that extend the internal electrodes outside thepiezoceramic stack thus form a noncontinuously structured, strip-shapedbase metallization, these rails or tabs being little stressed, if atall, by the mechanical motions of the adjacent piezoceramic films duringoperation of the actuator. Because these rails or tabs are electricallyinterconnected in mechanically flexible fashion, a particularly strongactuator can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of an actuator according to thepresent invention.

DETAILED DESCRIPTION

According to FIG. 1, a piezoceramic actuator 1 according to the presentinvention is substantially made up of a stack of sintered piezoceramicfilms 2 having arranged between them metallic internal electrodes 3which extend alternately to the right and left side of actuator 1 thatis depicted, i.e. are accessible from outside between the adjacentpiezoceramic films 2. On the respectively opposite edge region, eachinternal electrode is covered by the adjacent piezoceramic films 2, sothat the edge of the respective internal electrode 3 is inaccessiblefrom the outside.

By electrolytic metal deposition (presented below), rail-like extensions4, which for example are each made up of a nickel layer 4′ directlyadjacent to inner electrodes 3 and a gold layer 4″ located externallythereabove, are shaped onto the externally accessible edge regions ofinner electrodes 3 to the right and left in the drawing.

The exposed edges of rail-like extensions 4 are electricallyinterconnected via electrically conductive films 5′ and 5″ which aremade, for example, of plastic, e.g. silicone or copolymers, andelectrically conductive carbon or metal particles embedded therein; inorder to achieve the desired electrical conductivity, these particlesare very densely packed, and the plastic material serves substantiallyto ensure mechanical bonding of the particles.

Rail-like extensions 4 and films 5′ and 5″ can be interconnected inelectrically conductive fashion, for example, by hot pressing.

The two films 5′ and 5″ are in turn electrically connected to connectorlines 6′ and 6″ through which films 5′ and 5″, and thus internalelectrodes 3 electrically connected thereto, can be connected to anoperating voltage source, in such a way that the group of internalelectrodes 3 electrically connected to film 5′, and internal electrodes3 electrically connected to film 5″ and engaging in comb fashion betweenthe aforesaid internal electrodes 3, respectively have electricallyopposite polarities, and each piezoceramic film 2 located therebetweenis impinged upon by a corresponding electric field.

Depending on the polarity of the electrical operating voltage, the upperand lower ends of actuator 1 then perform motions relative to oneanother in accordance with arrow P.

Since films 5′ and 5″ are spatially separated from the edges ofpiezoceramic films 2, and since films 5′ and 5″ moreover possess acertain elastic flexibility, the motions of actuator 1 cannot cause anydamage to films 5′ and 5″.

Films 5′ and 5″ may also exhibit a corrugated structure, in such a waythat an externally convex ridge extends between each two internalelectrodes 3 attached adjacently to film 5′ or 5″, and between theirrail-shaped extensions 4.

Alternatively, it is also possible to replace the conductive films 5′and 5″ with metal knitted structures or meshes, or also with a layer ofmetal foam.

Electrochemical production of rail-shaped extensions 4 can beaccomplished as follows.

The stack of sintered piezoceramic films 2, having internal electrodes 3arranged therebetween, is immobilized in a holder. Internal electrodes 3are then electrically contacted to one another on the two opposite sides(in FIG. 1, depicted on the right and left sides of the stack), but insuch a way that the respective contacts still leave open a largercontinuous region of the mutually opposite sides of the stack.

This is followed by cleaning of the stack in a neutral cleaner, forexample at a temperature of 55° C. and a treatment time of five minutes.

A rinse in demineralized water is then performed.

Electrochemical metal deposition is now performed, for example a nickeldeposition or deposition of a nickel alloy from a nickel sulfamateelectrolyte which, in the case of deposition of an alloy, containscorresponding additives or alloy components. A noble metal depositionfrom a corresponding electrolyte can optionally also be accomplished.

During deposition, internal electrodes 3 are electrically connected asthe cathode via the aforesaid contacts of the stack, and a suitableanode is used.

The nickel sulfamate electrolyte can have a pH of between 3 and 4 and atemperature of approximately 40° C. Other electrolytes are operatedunder similar process conditions.

The electrical current intensity between cathode and anode can be 1mA/cm² referred to the exposed ceramic surface. With this, a depositionrate of approx. 0.1 μm/min is achieved. After the production of metallayers 4′, another rinse in demineralized water is performed.

A hard gold deposition in a gold electrolyte is then performed, internalelectrodes 3 again being connected as the cathode; an anode ofplatinum-plated titanium can be used. The pH of the gold electrolyte canbe set to a value of 4 to 5. The temperature can once again be 40° C.The current intensity can once again be 1 mA/cm² referred to the exposedceramic surface of the ceramic film stack.

Alternatively, a uniform gold layer approx. 0.1 mm thick can also bedeposited in electroless fashion from a hot gold electrolyte. Thetemperature for this method step can be between 80° C. and 90° C.

Another rinse in demineralized water is then performed.

Rail-like extensions 4 are now available for connection to theelectrically conductive films 5′ and 5″ or the like.

1. A piezoceramic actuator comprising: a monolithic stack of thinpiezoceramic films; and internal electrodes arranged between the films,the internal electrodes being electrically interconnected on outer sidesof the stack to form at least two electrode groups electricallyseparated from one another, the internal electrodes each having arail-like extension in a region of the outer side of the stack; whereineach rail-like extension has one of electrochemically depositednickel-alloy and nickel; and wherein each rail-like extension is made ofnickel-alloy layer and a gold layer.
 2. The piezoceramic actuatoraccording to claim 1, wherein at least one of metal knitted structures,metal meshes and metal foam electrically interconnecting exposed edgesof each rail-like extension at a distance from the outer sides of thestack.
 3. The piezoceramic actuator according to claim 2, wherein eachrail-like extension has one of electrolytically deposited nickel-alloyand nickel.
 4. A piezoceramic actuator comprising: a monolithic stack ofthin piezoceramic films; and internal electrodes arranged between thefilms, the internal electrodes being electrically interconnected onouter sides of the stack to form at least two electrode groupselectrically separated from one another, the internal electrodes eachhaving a rail-like extension in a region of the outer side of the stack;wherein each rail-like extension has one of electrochemically depositednickel-alloy and nickel; and wherein each rail-like extension is made ofnickel-alloy layer and an adjacent gold layer.
 5. The piezoceramicactuator according to claim 2, wherein the at least one of metal knittedstructures, metal meshes and metal foam is at least partially made fromplastic.
 6. The piezoceramic actuator according to claim 5, wherein theat least one of metal knitted structures, metal meshes and metal foam isat least partially made from at least one of electrically conductivecarbon or metal particles.
 7. A piezoceramic actuator comprising: amonolithic stack of thin piezoceramic films; and internal electrodesarranged between the films, the internal electrodes being electricallyinterconnected on outer sides of the stack to form at least twoelectrode groups electrically separated from one another, the internalelectrodes each having a rail-like extension in a region of the outerside of the stack; wherein each rail-like extension has anelectrochemically deposited nickel-alloy.